The fixed-function pipeline, written out

The left image is drawn by the graphics hardware. The middle one is drawn by a fragment shader that does the hardware's job by hand — clipping, culling, subpixel snapping, edge functions, the depth test, perspective-correct interpolation. The right one is the difference between them, amplified eight times. Untick a stage to see what the hardware was doing for you.

hardware pipeline

software rasteriser

difference (x8)

measuring…

pipeline stages near/far clipping backface culling depth test perspective-correct interpolation texturing subpixel snapping

scene cube and ground foreshortened quad interpenetrating quads near-plane straddler shared edge opposed windings far plane

camera far plane at 20.0 pull the plane in and anything beyond it is clipped away. The range follows the scene, because the far-plane scene sits at depth 45–55 and the rest are within 12 of the camera.
Unticking near/far clipping on its own will not bring the far geometry back: the depth buffer clears to 1.0 and the test is a strict <, so anything at or beyond the far plane is rejected a second time on depth alone. Untick depth test as well to see it drawn. The hardware does the same thing, which is why a triangle sitting exactly on the far plane renders nothing in either path.